1. Field of the Invention
The present invention concerns a method for correction of movement in the acquisition of MR (magnetic resonance) images and an MR system for this. The invention can in particular (but not exclusively) be used in the acquisition of MR images of the head. The invention can be applied in any other body region that is intrinsically rigid but is possibly subject to a translation or rotation movement.
2. Description of the Prior Art
In imaging by means of magnetic resonance, movement of the examination subject during the signal acquisition typically leads to artifacts in the image that are more or less noticeable in a disruptive manner in the interpretation of the MR images. Such movement artifacts can include artifacts due to movement of the examination subject that result in the acquisition of a single MR image (i.e. one slice). This is subsequently classified as an intra-slice movement that results given the acquisition during one slice. In addition to this intra-slice movement, inter-slice movement can occur, which is movement that occurs in the time span that arises between the acquisition of various slices at different locations. For example, movement in the acquisition of a single slice leads to signal loss, signal smearing in the image, or to what are known as ghost artifacts. Various methods exist for correction of these movement artifacts. For example, translation and rotation movements of a rigid body that occur in an image plane are suppressed in the imaging, as is described (among other things) by James G. Pipe in “Motion Correction with Propeller MRI: Application to Head Motion and Free-Breathing Cardiac Imaging”, Magnetic Resonance Medicine 42:963-969, 1999.
Inter-slice movements influence the acquired MR images such that the acquired anatomy in adjacent slices is not aligned on one another but rather is shown offset or rotated by an angle. Since the physician who “leafs through” the images normally examines the images in anatomical order given exposures in multiple slices, a translationally offset slice or a slice rotated by an angle is very disruptive for the assessing physician.
A theoretical possibility to reduce such movement artifacts is to reduce the time interval in the acquisition of two adjacent slices, but this has the following disadvantage. Given the excitation of adjacent slices in short succession, a perfect, rectangular slice profile is not always excited by a selected radio-frequency excitation pulse (RF pulse). Each RF excitation pulse thus unavoidably also influences the slices adjacent to the excited slice. If the adjacent slices are now themselves excited before the excitation induced by the adjacent slice has decayed, the contrast is altered. One way around this is an acquisition method in which the slices are not excited one after another in their anatomical order; but instead, for example, one slice is respectively skipped. In this case this means that only every other slice is excited while the previously omitted slices are excited at a later point in time. Such acquisition methods afford the possibility to either first entirely acquire all slices of a first group (i.e. for example the slices 1, 3, 5, . . . ) and afterwards the remaining slices of a second group (i.e. the slices 2, 4, 6, . . . ). The complete acquisition of a slice thus generally requires multiple excitations. As an alternative, the slices of the first group and the second group can be acquired interleaved with one another, wherein after the acquisition of a first part of the first slice and before a further excitation of the first slice, a first part of the adjacent slice of the second group is acquired within the repetition time TR, for example. The repetition time TR is approximately 4-12 seconds given T2-weighted exposures of the head. In the last cited example of the interleaving, the time interval between the excitation of adjacent slices is approximately TR/2 (i.e. 2-6 seconds). For the other acquisition schemes in which one group is first entirely acquired before the next group is acquired, the time interval between corresponding excitations of two adjacent slices lies in the minute range. In both acquisition techniques, however, the time span that elapses between the acquisition of adjacent slices is not negligible. In the event that the examination region moves within this time span, this leads to a shift of the shown examination region in the image.
Techniques in order to reduce movement artifacts that arise during the acquisition of a slice are known in the prior art (see the preceding aforementioned publication by James G. Pipe, “Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) MRI; Application to Motion Correction”, MRM 42:963-969, (1999)). Such intra-slice movement corrections, however, cannot be used for correction of movement artifacts that result from movement between the acquisition/excitation of various anatomical slices.